1. Field of the Invention
The present invention relates to an image sensor and an image capturing apparatus.
2. Description of the Related Art
A digital camera, which aims at capturing still images and moving images, normally uses a CCD sensor as an image sensor so far. However, in recent years, performance of a CMOS sensor has been remarkably improved, and substitutions from a CCD sensor have progressed in some cameras. As one of features of a CMOS sensor, each pixel has an active element. As this effect, each pixel can convert charges into a voltage signal, and it is known that smear, which is a problem in a CCD sensor and is generated upon capturing a high-luminance object, can be greatly reduced. However, as a negative effect, a light-receiving area cannot be broadened, and a numerical aperture decreases, thus posing a problem. In order to solve this problem, a method of sharing an active element, which is arranged for each pixel in a conventional CMOS sensor, by a plurality of pixels to broaden a light-receiving area (Japanese Patent Laid-Open No. 9-46596) has been proposed.
However, by increasing the number of pixels which share an active element so as to broaden a light-receiving area, symmetry for each pixel is unwantedly collapsed, thus posing another problem. It is known that this problem results in a drop of optical characteristics such as shading characteristics and spectral characteristics. Furthermore, since a layout difference of, for example, wiring lengths between sharing pixels is generated, charge transfer characteristics undesirably drop.
Conventionally, since the influence on image quality caused by the charge transfer characteristic drop is minimal, it is not seen as a problem. However, as low-noise and high-sensitivity requirements for image sensors used in recent digital cameras become stricter, the influence on image quality caused by poor symmetry can no longer be ignored.